(a) Field of the Invention
The present invention relates to the fabrication of a semiconductor pn junction, and more particularly, it pertains to a method of fabricating a pn semiconductor junction by the use of Group IIB-VIB compound semiconductors.
(b) Description of the Prior Art
Group IIB-VIB compound semiconductors, such as ZnS and ZnSe, are semiconductors having a relatively large energy band gap. Thus, if it is possible to form a good pn junction by the use of a semiconductor material selected from Group IIB-VIB compound semiconductors, the resulting devices will have novel functions and especially light-emitting diodes (hereinafter referred to as LED or LEDs) which can emit light in a wavelength region whose peak intensity of light emission is shorter than about 5500.ANG.. In terms of color such a light emission ranges from blue-green to blue-violet.
However, as is well known with many of the Group IIB-VIB compound semiconductors, such as ZnS and ZnSe, the resulting crystals have only an n type conductivity. The art has been unable to produce stable p type semiconductor crystals except ZnTe.
The conductivity types and the energy band gaps of typical currently available Group IIB-VIB compound semiconductors are shown in the Table below.
TABLE ______________________________________ Semiconductor material ZnS ZnSe ZnTe CdS CdSe CdTe ______________________________________ Conductivity type n n p n n n p Energy band 3.6 2.8 2.2 2.5 1.74 1.5 gap(eV) ______________________________________
First an n type semiconductor crystal will be described. N-type crystals, including those having a low resistivity, are relatively easy to obtain in the IIB-VIB compounds with a narrower band gap. They can be obtained by doping a Group III or Group VII element which can serve as an n type impurity. However, in the past it has been impossible to obtain a stable p type semiconductor crystal by doping an impurity by relying on the same impurity-doping technique as above described. More precisely, even when: (1) an impurity serving as an acceptor is doped in order to obtain a p type crystal, or more concretely even when an impurity is doped from liquid phase during the solution growth to form a p type semiconductor, (2) when an acceptor impurity is doped into an n type semiconductor crystal by relying on the diffusion technique to thereby convert the conductivity type of a portion of such n type crystal into a p type, (3) it is intended to convert the conductivity type of a portion of a semiconductor crystal into a p type by relying on the alloy method, the resulting crystal either still remains the original n type, or it becomes a crystal having a high resistivity to approximate that of an insulator. Thus, practically useful p type semiconductors have not been obtained, hence it has not been possible to fabricate a good pn junction.
The problem of crystallinity of the crystal obtained is considered to be the main reason for this. Therefore, the following description will refer to the growth of a Group IIB-VIB compound crystal by selecting a ZnSe crystal as an example.
Conventional procedures for manufacturing substrate crystals such as ZnSe have required a high temperature of about 1500.degree. C., for example, in Bridgman's method. The vapor transport method also requires a high temperature of 1000.degree. C., or even higher. Thus, these conventional methods give rise to the development of a very large deviation from stoichiometry. The more the Group VI element atoms such as Se escape from the crystal, the more the crystal will tend to become an n-type crystal. If there is a large deviation from stoichiometry, the vacancies will easily combine with the residual impurities located within the crystal, or with the impurities which are later introduced into the crystal during a subsequent process. Thus, the number of vacancies is hard to control, and the presence of such vacancies further complicates the later formation of a pn junction. Thus, it is quite desirable to form a substrate crystal at as low a temperature as possible.
The reason the art has not been able to obtain a good p type semiconductor crystal with a Group IIB-VIB compound semiconductor is because of the fact that, when an acceptor impurity is introduced in order to obtain a p type semiconductor crystal, there develop, within the crystal, defects which function as donors in accordance with the amount of the acceptor impurity which is introduced in the crystal as a natural trend to thermo-dynamically establish stability. This develops acceptor carrier compensation.
Development of acceptor carrier compensation as described is called a "self-compensation phenomenon". Those defects developing within the crystal which function as the donor consist of, for example, Se vacancies which develop due to the escape of Se atoms from the crystal ZnSe, this escape taking place because, in such compound semiconductors as ZnSe, Se atoms have a vapor pressure higher than Zn atoms, and because, for this reason, Se atoms easily escape from the ZnSe crystal. These defects also include a complex of Se vacancies with the doped impurity. These defects, i.e. Se vacancies and/or complexes with Se vacancies and impurities, function as donors.
This effect holds true not only for ZnSe, but also for ZnS as well. The above description has been made only on those Group IIB-VIB compounds as will provide n-type crystals. Other compounds, such as ZnTe which provide only a p type crystal, and CdTe which provides crystals of both n-type and p-type conductivities, are not included in the present invention.
In particular, ZnSe and ZnS, which are Group IIB-VIB compounds containing Zn among their component elements, have wide forbidden band gap, and therefore they are important materials for a blue-LED which cannot be made using Group III-V compounds. Thus, the appearance of a p-n junction having good characteristics is keenly desired.